Such drills are well known and are used in preference to drills having electrical drives both because of their power/weight ratio and also for safety reasons. In known drills, a single pneumatic motor serves both to apply rotary drive to a spindle and also, via a lead screw and nut, to feed the same spindle in translation.
The drawback of such drills results from the fact that, in practice, they are capable of rotating, and consequently of feeding, at one speed only, as determined by the speed of rotation of the pneumatic motor. The speed of rotation of the spindle and its feed rate can naturally be altered by exchanging an existing set of gears for another set that provides a different speed ratio. Thus, a single drill needs to have several motors and several sets of gears in order to change between types of application, e.g. to go from a feed rate of 0.1 millimeters per revolution (mm/rev) to 0.3 mm/rev, which involves assembly and disassembly, or else having numerous drills.
Unfortunately, and particularly in aviation, it is becoming more and more common to use composite materials made up of layers of different materials, and in particular aluminum-carbon-titanium laminates. Each of these materials requires different working conditions. This gives rise to problems of surface state, accuracy, and/or cycle time.